Mounting an RFID tag within a plastic cap for a container, e.g., a beverage bottle, has presented no difficulty since the plastic material does not significantly affect the transmission of the electromagnetic signal transmitted to the RFID tag.
However, the use of an RFID tag with a metal container closure or cap present certain design difficulties. As used herein, metal cap is understood to mean any metal closure for any type of container. Furthermore, references herein to bottles and metal caps for bottles is not to be understood as limiting the scope of the invention but merely illustrative of a particular application for the invention. At the high RF frequencies used for communication with an RFID tag, some transmitted signal energy will diffract and reflect into a metal cap from the open end of the metal cap so long as the fluid contents within the container remain below the bottom of the cap. However, a full container will likely prevent the RF signal from reaching an RFID tag mounted within a metal cap. Furthermore, since an RFID tag normally does not include an integral battery and is powered by the received RF energy, sufficient RF energy has to reach the RFID tag to power the integrated circuit chip on the RFID tag. It is unlikely that this would occur for an RFID tag mounted within a metal cap absent special circumstances, such as positioning the interrogator antenna at a very close range and at a specific orientation to the metal cap. Consequently, a conventional RFID tag mounted completely inside a metal cap does not appear to be practical.
Microstrip antenna technology originated in microwave transmission lines etched into radio frequency integrated circuits and into copper-clad printed circuit boards. A microstrip transmission line is a metal conductor path (usually etched copper) separated from an expansive conducting surface (ground plane) by an insulating dielectric layer. The width of the transmission line and the thickness of the dielectric medium determine the characteristic impedance of the transmission line, and thereby the efficiency of RF power transmission from one device to another. If the length of the microstrip transmission line is adjusted to be one-half the wavelength of RF waves in the dielectric layer, and if one or both ends of the transmission line are not connected to a device, then that transmission line radiates energy (or receives it) as an antenna. Consequently, the same technology and the same process steps can be used to produce an antenna and the necessary impedance matching components, resulting in lower manufacturing costs.
For these reasons, microstrip antennas are commonly used in connection with the interrogator of a RFID system. These antennas have the desirable characteristic of laying flat on a surface with minimum protrusion from that surface. However, they are not commonly used on RFID tags, primarily for the following three reasons: 1) The characteristic length of a simple microstrip antenna is one-half of the wavelength, whereas it is one-quarter of the wavelength for an electric dipole antenna. Consequently, for a given frequency of operation, the microstrip antenna must be twice the length the electric dipole antenna. 2) The simplest microstrip antennas have a narrower bandwidth than the electric dipole antenna, resulting in tighter manufacturing tolerances for the microstrip antenna. 3) Since the patch of the microstrip antenna is more massive than the wire antenna, the RFID tag IC chip must have more substantial power conversion and switching devices than is necessary for the wire antenna in order to modulate the backscattered RF energy return to the interrogator.
The use of a microstrip antenna for an RFID tag has been disclosed in U.S. Pat. No. 6,215,402, which includes several designs for patch antennas and impedance matching components for an RFID tag, and U.S. Pat. No. 6,329,915, which describes the use of an additional insulating material with high electric permittivity that is applied to the surface on top of the microstrip antenna in order to further reduce the size of the antenna. However, neither of these patents discloses the use of an RFID tag having a microstrip antenna on a metal closure for a container.
The use of specially designed slots etched into the interior of a patch antenna to broaden the bandwidth of a microstrip antenna without changing the overall form factor of the antenna is disclosed in an article by Ali, Sittironnarit, Hwang, Sadler, and Hayes, entitled “Wideband/Dual-Band Packaged Antenna for 5–6 GHz WLAN Application,” that appeared in the February, 2004 issue of the journal IEEE Transactions on Antennas and Propagation. However, this article does not disclose the use of an RFID tag having a microstrip antenna on a metal bottle cap.
Accordingly, it is an object of the present invention to provide an RFID tag employing an antenna that can be mounted on the exterior of a metal closure for a container and that provides the same functionality as a conventional RFID tag mounted on a plastic closure for a container.
It is a further object of the present invention to provide an RFID tag for mounting on a metal cap that is not subject to close tolerances in manufacturing.